Tin plating



Patented Aug. 13, 1940 UNITED STATES 2,211,251 I TIN PLATING Harold J.Barrett;

com-hams.

Niagara Falls, Wernlund, North Tonawanda, N. Y., assignors to E. I. duPont de Nemours& Company, Wilmington, Del., a corporation of Delaware NoDrawing. Application November 23, 1938, stl'lll N0. 242,.

140mm; wiser-11) r depositing tin from stannite-stannate baths of v Thisinvention relates to the art of electroplating metals and moreparticularly to a novel addition agent for electroplating bathscontaining bivalent tin, more particularly alkaline electroplatingbaths, to improve the qualities of the electroplated deposits.

This applicaiionis a continuation-impart of our copendingapplicationSer. No. 90,382, filed July 13, 1936.

In the art of electroplating metals with tin from alkaline baths it isknown that thepresence of bivalent tin ions tends to lead to the formaltion of spongy or porous deposits. Heretofore,

therefore, tin plating baths were almm exclusively prepared from tin inits quadrivalent form. The deposits obtained from such stannate bathspossess a very fine grain structure, the fineness of which still can befurther enhanced by the addition of corn sugar, starch or the likeaddition agents to the plating bath, This electroplating process is alsocharacterized by its good throwing power, i. e. the tin deposited fromsuch stannate baths is evenly deposited on articles of any kind, both onthe inner and outer surfaces. The current efllciency of these knownprocesses is about based on the deposition of Sn++++ ions. Although thisprocess leads to highly satisfactory results as to the quality of theproducts obtained, it leaves much to be desired with respect to the'current efliciency and to the stability ofthe stannate during theelectroplating process.

The weight of tin deposited from stannate baths at a given currentdensity is only half'as great as that from stannate baths. Besides this,usually a slight amount of the stannate is reduced to stannite and it isknown that even small amounts of stannite in a stannate plating bath maylead to the formation of spongy and porous deposits. To overcome thiscondition hydrogen peroxide or other oxidizing agents may be added fromtime to time to such stannate plating baths to oxidize any stannitesformed. Although by this expedient the formation of stannite isprevented the cost of the plating baths is increased thereby. Thisdisadvantage as well as the rela-'- tivelylow rate of metal depositionis not very serious where only thin deposits are required. However, forthe formation of heavy deposits a higher speed of metal deposition isurgently needed to decrease the time of plating.

The object. of the present invention is an improved electroplatingprocess for depositing metals from plating baths containing tin in thebivalent form. Another object of the invention is varying proportions. Aiurther object of the invention is an improved process for depositingtin from straight stannite plating baths. A further object of theinvention is a new addition agent for electroplating baths containingbivalent tin ions. Further objects :of the invention will be hereinafterapparent.

The above objects of the invention are accomplished by adding to anddissolving in a plating bath containing bivalent tin ions or tending tothe formation of such bivalent tin ions a hydrophilic' polyvinylcompound selected from the group consisting of polyvinyl alcohol,polyvinyl esters, polyvinyl ethers and polyvinyl acetals. By dissolvingsuch ahydrophilic polyvinyl compound in a plating bath containingbivalent tin ions, the formation 01' spongy or porous deposits isinhibited or substantially prevented. Fbr example, when adding a smallamount of such a hydrophilic polyvinyl compound to an alkaline stanniteplating'bath, deposits of a substantially increased whiteness and apleasing matte luster are obtained. Thus it is made possible to deposittin from straight stannite plating baths and thereby to obtain twice thethickness of plate under a given set of conditions in a given time aswhen using the stannate bath. Although the throwing power of suchstannite baths is somewhat inferior to the throwing power of stannatebaths, such stannite baths will be preferable wherever articles ofsimple shape such as metal sheets are coated or where low costs or highpower eiliciencies, respectively, or thick coatings are essential.

When the polyvinyl addition agents are added to a stannate bath thedeleterious efiect on the deposits of small amounts of stannite presenttherein or formed during the plating process is eliminated and theplating bath will retain substantially the characteristics of a purestannate bath. By this expedient therefore the addition of hydrogenperoxide or other oxidizing agent to the stannate bath is madeunnecessary.

Our invention makes it further possible to deposit tin from platingbaths containing any ratio of stannate and stannite. The properties ofthe resulting deposits will be governed by the ratio of the stannate tostannite used in the bath, the

Our invention is accordingly applicable wherever bivalent tin ions arepresent or are formed 55 'taining bivalent tin ions.

regardless of the specific composition of the plating bath. Ourinvention accordingly is not restricted to any specific composition ofthe plating bath but in its broad aspect is directed to a platingprocess or plating bath, respectively, where bivalent tin ions arepresent either in small or in large amounts. Preferably, our inventionis utilized for alkaline plating baths con- A large number of tinplating baths are known which are satisfactory but for the presence ofstannite and it will be self-evident for anyone skilled in the art ofelectroplating where to apply our invention. Our invention is likewiseapplicable for plating tin alloys where the plating bath contains one ormore metals other than tin but tends to lead to the formation of spongyor porous deposits due to the presence of bivalent tin ions, and wherethe herein described polyvinyl addition compounds are suitable forelectroplating such other metals. Also our method obviously is suitablefor plating from stannite plating baths which contain minor .amounts ofany other metal.

The group of addition agents of our invention includes a variety ofhydrophilic polyvinyl compounds selected from the group of polyvinylalcohol, polyvinyl esters, polyvinyl ethers and polyvinyl alcoholacetals which all are more or less soluble in water. As used in thespecification and the appended claims the terms "soluble in water and"in active solution and similar expressions are meant to indicate thatthe hydrophilic polyvinyl compounds are capable of forming truesolutions in water or colloidal suspensions (hydrosols) in water.Polyvinyl alcohol itself is a resinlike material which may be obtainedby hydrolysis of polyvinyl acetate which in turn is made by polymerizingvinyl acetate. Polyvinyl alcohol may be made in a number of grades ormodifications which differ one from another by their degree ofpolymerization. All of these modifications are fairly soluble in water,for example, 10% by weight aqueous solution of polyvinyl alcohol mayreadily be made although the actual solubility will vary somewhatdepending upon the degree of polymerization. Various derivatives ofpolyvinyl alcohol may be made by different chemical reactions, whichderivatives may be either water soluble or capable of being swelled bycontact with water. We use the term hydrophilic polyvinyl compoundsselected from the group consisting of polyvinyl alcohol, polyvinylesters, polyvinyl ethers, and polyvinyl alcohol aicetals" hereinafterand in the appended claimsfigto designate such materials made from orderived from polyvinyl alcohol which are soluble in water in theeifective amount or are capable of forming colloidal suspensions inwater. One example of such hydrophilic polyvinyl compounds of the abovespeci fled group is the so-called partial ester of polyvinyl alcoholwhich may be made by only partially hydrolyzing a polyvinyl ester. Ifthe hydrolysis of the ester is carried out to a sumcient extent, theresulting compound which contains both ester groups and free hydroxygroups in the same molecule will be hydrophilic in nature. Otherderivatives of polyvinyl alcohol having similar properties are thepartial polyvinyl alcohol acetals which may be made by incompletelycondensing polyvinyl alcohol with aldehydes such as formaldehyde oracetaldehyde and the partial ethers of polyvinyl alcohol which may bemade by partially etherifying polyvinyl alcohol with some other alcoholsuch as glycerine, methanol or the like. Also certain esters ofpolyvinyl alcO- hol may be made which are sufficiently watersoluble forthe purposes of the present invention, for example, the boric acid esterand the sulfuric acid ester; the latter ester may be made by reactingpolyvinyl alcohol with sulfuric acid.

The amount of hydrophilic polyvinyl compounds to be added to theelectroplating bath in general will vary, depending upon the compositionof the plating bath, and the degree of whiteness required. Very smallamounts of these materials bring about the desired effect and as amatter of fact almost any amount dissolved in the plating bath willbring about an improvement in the deposits. However, additions of suchhydrophilic polyvinyl compounds greater than the equivalent of one gramper liter of polyvinyl alcohol usually will not bring about any furtherimprovement of the desired effect and therefore will not be utilized incommercial plating work. As a rule, the upper limit will be determinedby the solubility of the polyvinyl compound used and additions inamounts which lead to precipitation should be avoided since precipitatesin plating baths generally are undesirable.

We have found that for the application of our invention for platingbaths which consist preponderantly of stannite solutions, optimumresults can be obtained when observing specific conditions. Thus, forelectroplating metals in plating baths substantially composed of alkalimetal stannite containing from about 13 grams per liter to 104 grams perliter of sodium stannite and from about 10 grams per liter to 65 gramsper liter of sodium hydroxide and from about 0.1 to 1 gram per liter ofpolyvinyl alcohol are especially suitable. Equivalent amounts of othersoluble stannites and other alkaline agents and other hydrophilicpolyvinyl compounds can be used. Cathode current densities up to 30amperes per sq. ft. can be advantageously utilized when keeping thebaths at about C. Further increase of current density may lead to theformation of dark deposits.

The sodium stannite solutions, for example, can be readily prepared byadding a stannous chloride solution slowly to a caustic soda solutionwith thorough stirring. The temperature should not exceed 20 C. Ifdesired, the slight amount of precipitate which is formed may befiltered off. Especially suitable solutions can be prepared by mixingstannous chloride and caustic soda within the following range:

Grams per liter SIJCh-2Hz0 15 to NaOH- 15 to Polyvinyl alcohol 0.1 to 1Preferably we use 110 grams per liter of Such-211:0 and 150 grams perliter of NaOH and 0.2 gram per liter of polyvinyl alcohol. The cathodiccurrent density for different temperatures preferably is maintained asfollows:

30 C.--not more than 7 amp/sq. ft. 65 C.-not more than 15 amp/sq. ft. 80C.--not more than 30 amp/sq. ft.

tic soda on the above concentrations to 50 grams per liter lowers thecurrent density range to about up to 10 amperes per sq. ft. Theinvention is further illustrated by the following examples:

Example 1 One liter of a solution containing 220 grams of stannouschloride per liter of water was slowly added to one liter 01' a cooledsolution of sodium hydroxide containing 300 grams per liter and thetemperature was carefully held below 20 0. Small amounts of precipitatewere filtered off. The sodium chloride formed by this reaction had noinfluence whatsoever on the subsequent electroplating process andtherefore was not removed from the plating bath. To this solution 0.2 to4 grams per liter of polyvinyl alcohol was added and dissolved therein.

This bath was then utilized for the deposition of tin on a steelsurface, utilizing pure tin as anode and clean sheet steel as cathode.Equally good results were obtained with varying amounts of polyvinylalcohol in the above specified range.

For the first 8 hours a current density of about 30 amperes per sq. it.at 80 C. was applied and during the second 8 hours the current densitywas lowered to 15 amperes per sq. ft. The plated articles had a densetin coating of a pleasing white color and a matte luster. The surface ofthe deposits as compared with the surface of deposits obtained from astannate bath had a somewhat coarser grain.

Example 2 Equal volumes of the following solutions were prepared andmixed together.

The resulting bath was operated in the temperature range of 70 to 80 C.to plate tin onto a steel cathode. For the first hour 30 amperes per sq.It. was utilized and thereafter about 20 amperes per sq. ft. A smallamount of tin oxide precipitated out of this bath but this did notaifect the quality of the tin plate produced. The electroplated articleswere of similar appearance to those described in Example 1.

Example 3 The following plating baths were prepared and operated at '70to 80 C.

#3 #4 Plating bath Grams per Gra ns pcr Grams per Grains per liter literliter liter SnCl1-2H20 30 l5 15 15 NaOH 30 15 20 15 Polyvinyl alcohol0.2 0.3 0. 2 0.2

Bath #1 5 to 20 Bath #2 5 to 15 Bath #3 5 to 15 Bath #4 5 to 10 Tinsalts other than tin chloride and soluble stannites other than sodiumstannite can be used for the process of our invention. As stated abovethe process of our invention is applicable for any electroplating bathcontaining stannite vinyl compound selected from the group consisting ofpolyvinyl alcohol, polyvinyl esters, polyvinyl ethers, and polyvinylacetals.

3. An electroplating process which comprises electroplating metals withtin in an alkaline plating bath containing a soluble stannite and havingdissolved therein a hydrophilic compound selected from the groupconsisting of polyvinyl alcohol, polyvinyl esters, polyvinyl ethers, andpolyvinyl acetals.

4. An electroplating process which comprises electroplating metals withtin in an alkaline plating bath consisting preponderantly of a solublestannite and having dissolved therein a hydrophilic compound selectedfrom the group consisting of polyvinyl alcohol, polyvinyl esters,polyvinyl ethers, and polyvinyl acetals.

5. An electroplating process which comprises electroplating in analkaline plating bath containing a soluble stannite and polyvinylalcohol.

6. An electroplating process which comprises electroplating metals withtin in an alkaline plating bath containing an alkali metal stannite andhaving dissolved therein polyvinyl alcohol.

'7. An electroplating process which comprises electroplating metals withtin in an alkaline plating bath consisting preponderantly of alkalimetal stannite and having dissolved therein polyvinyl alcohol.

8. An electroplating bath containing a soluble stannite and ahydrophilic compound selected from the group consisting of polyvinylalcohol. polyvinyl esters, polyvinyl ethers, and polyvinyl acetals.

9. An electroplating bath comprising alkali metal stannite and alkalimetal hydroxide and containing a hydrophilic polyvinyl compound selectedfrom the group consisting of polyvinyl alcohol, polyvinyl esters,polyvinyl ethers and polyvinyl acetals.

10. An electroplating bath comprising alkali metal stannite and alkalimetal hydroxide and containing polyvinyl alcohol.

11. A tin plating bath comprising sodium .0

stannite, sodium hydroxide and a hydrophilic polyvinyl alcoholderivative, prepared from a mixture of 15 to grams per liter stannouschloride, 15 to grams per liter sodium hydroxide and 0.1 to 1 gram perliter polyvinyl alcohol.

12. A tin plating bath containing alkali metal stannite and stannate anda hydrophilic'polyvinyl compound selected from the group consisting ofpolyvinyl alcohol, polyvinyl esters, polyvinyl ethers, and polyvinylacetals.

13. The process comprising preparing alkaline tin plating bath bydissolving together in water a stannous salt, alkali metal hydroxide anda hydrophilic polyvinyl compound selected from water stannous chloride,sodium hydroxide and the group consisting oi polyvinyl alcohol,polypolyvinyl alcohol and thereafter electrolyzing vinyl ester,polyvinyl ethers and polyvinyl acetals said bath. and thereafterelectrolyzing said bath. HAROLD J. BARREIT.

5 14. The process comprising preparing an al- CHRISTIAN J. WERNLUND. 5

kaline tin plating bath by dissolving together in

